Piezoelectric/electrostrictive devices, which are also called film type elements, have conventionally been used as actuators or various kinds of sensors. A piezoelectric/electrostrictive device used as a sensor is used for measuring characteristics such as density, concentration, and viscosity of fluid as disclosed in the Patent Document 1. Such a piezoelectric/electrostrictive device is used as a sensor by utilizing a correlation between an amplitude of a transducer being provided in the device and a viscous resistance of fluid in contact with the transducer. Since a vibration mode in a mechanical system such as vibrations of a transducer can be converted into an equivalent circuit in an electric system, when the piezoelectric/electrostrictive device is caused to be vibrated in fluid so that the transducer receives a mechanical resistance based on a viscous resistance of fluid, a change in an electric constant of an equivalent circuit of a piezoelectric body constituting the transducer is detected, which enables measurement of characteristics such as viscosity, density, and concentration of the fluid.
Fluids which can be measured by the piezoelectric/electrostrictive device as a sensor are liquid and gas. The liquid includes not only liquid having only one component such as water, alcohol, oil, or the like, but also liquid, slurry, and paste prepared by dissolving, mixing, or suspending a soluble or insoluble solute in the above liquid. Examples of the electric constant to be detected include a loss coefficient, phase, resistance, reactance, conductance, susceptance, inductance, electrostatic capacity (capacitance) and the like. In particular, a loss coefficient or a phase each having the minimum or maximum changing point at around a resonance frequency of the equivalent circuit is preferably employed. This enables measurement of not only viscosity but also density and concentration of fluid. For example, sulfuric acid concentration in a sulfuric acid aqueous solution can be measured. Incidentally, a change in the resonance frequency can be used as an index for detecting a change of a vibration mode besides an electric constant as long as no problem is caused in view of measurement accuracy and durability.
A piezoelectric/electrostrictive material is used for a piezoelectric/electrostrictive element, which corresponds to a portion where displacement is (vibrations are) caused in the piezoelectric/electrostrictive device. The piezoelectric/electrostrictive material generally has a ferroelectric phase at ordinary temperature and a paraelectric phase or an antiferroelectric phase at high temperature. For example, a piezoelectric/electrostrictive material containing Pb(Zr0.54Ti0.46)O3 (lead zirconate titanate) as the main component has a ferroelectric phase from ordinary temperature to around 320° C. and a paraelectric phase at around 320° C. or higher. In addition, for example, a piezoelectric/electrostrictive material containing (Bi0.5Na0.5)TiO3 (bismuth sodium titanate) as the main component has a ferroelectric phase from ordinary temperature to around 200° C., and an antiferroelectric phase from around 200° C. to around 320° C., and a paraelectric phase at around 320° C. or higher.
In the case of using such a piezoelectric/electrostrictive material for a piezoelectric/electrostrictive element, generally, after a piezoelectric/electrostrictive material, an electrode material, and the like are formed into desired shapes, followed by a heat treatment for integration, a polarization treatment is performed at temperature lower than the phase transformation temperature, where a ferroelectric phase at around ordinary temperature becomes the main phase, to obtain desired piezoelectric/electrostrictive characteristics. On this occasion, an electric constant, for example, a value of the electrostatic capacity of the piezoelectric/electrostrictive element after the polarization treatment is different from that before the polarization treatment. This is because crystal axes of the piezoelectric/electrostrictive material are oriented by the polarization treatment. Generally, a change ratio of the electric constant of the piezoelectric/electrostrictive element after the polarization to that before the polarization shows an almost constant value in each piezoelectric/electrostrictive material. In addition, a piezoelectric/electrostrictive element subjected to the polarization treatment can be restored in a state before the polarization treatment is conducted by a regeneration treatment where the piezoelectric/electrostrictive element is heated at the phase transformation temperature or higher. After the regeneration treatment, the electric constant of the piezoelectric/electrostrictive element returns to the value before the polarization treatment. When the polarization treatment is performed again, the electric constant becomes almost the same as that after the first polarization treatment. Thus, by the polarization treatment and the regeneration treatment, the electric constant of the piezoelectric/electrostrictive element can reversibly be changed.    Patent Document 1: JP-A-8-201265    Patent Document 2: JP-A-2004-23688